Paper i proceeding, 2011

Tungurahua is a steep-sided (5023 m high) andesitic stratovolcano located in the southern part of the Ecuadorian volcanic arc, with its summit crater lying just 8 km to both the touristic town of Baños and to a hydroelectric power plant that supplies 13% of the country’s total electric energy. Activity at Tungurahua resumed in September 1999 after 74 years of quiescence and since this time, the volcano has exhibited cyclic phases of activity. Typically, periods of high explosivity often characterized by strombolian to vulcanian behavior alternate with episodes of low to non explosive activity characterized by passive degassing.
The monitoring network deployed at Tungurahua is comprehensive, allowing an extensive record of seismic events, acoustic energy, SO2 emissions and changes in deformation, to be recorded. In July 2006, five broad-band seismic stations with accompanying infrasound receptors were installed, allowing seismic and acoustic energy to be calculated for each discrete explosion event. As well, SO2
fluxes have been recorded continuously since 2004 using two DOAS stations, before being enhanced to three sites in 2007. This complete record of both acoustic energy and SO2 fluxes subsequently allows meaningful correlations between the two parameters to be detected. Two clearly different kinds of behavior were distinguished:
1.- From July 2007 to August 2008: degassing was most of the time above the normal background levels of passive degassing (1000 ton/day). During this corresponding period, explosive activity was also continuous. Total SO2 released between July 2007 and October 2008 was in the region of 328 kton, giving a daily average emission of 960 ton/day, which was associated with a cumulate mean
acoustic energy of 93 kPa, which daily average is 247 Pa.
2.- From August 2008 to the present: high SO2 emissions are related to explosive activity.
Passive SO2 degassing which corresponds to approximately 65% of the time (during this period) reaches about 247 kton (456 ton/day) and is related to zero acoustic energy production. In contrast, explosive SO2 degassing reaches a total release of 456 kton (1,534 ton/day) and is related to a cumulate mean acoustic energy of 137 kPa, with a daily average of 508 Pa.
These trends suggest that before August 2008 magma levels at Tungurahua may have been closer to the surface or that conduit geometry was more favorable for efficient degassing showing moderate explosive activity, displaying conditions reminiscent of an open-vent system. Since this time however, volcanic behavior appears to alter to a more closed or blocked conduit regime with degassing efficiency becoming appreciably reduced. During such activity, explosive behavior dominates within well defined eruptive episodes, characterized by more energetic explosions with associated pyroclastic flows and higher SO2 fluxes, such as May and December 2010 eruptive events.